1. Field of the Invention
The present invention relates to an exposure method and apparatus, which are used in the manufacture of, e.g., a semiconductor integrated circuit or a liquid crystal display device.
2. Related Background Art
When a circuit pattern of, e.g., a semiconductor element or a liquid crystal display element is manufactured in a photolithography process, an exposure apparatus (e.g., a stepper) for exposing a photomask or reticle (to be referred to as "reticle" hereinafter) pattern on a photosensitive substrate is used. The photosensitive substrate is prepared by coating a photosensitive material such as a photoresist on a substrate such as a semiconductor wafer or a glass substrate, and the reticle pattern is transferred on the photosensitive material. In this case, since the photosensitive material has a predetermined proper exposure amount, the conventional exposure apparatus makes the following control, so that the exposure amount on the photosensitive substrate becomes the proper exposure amount.
A reticle having an actual circuit pattern or a test reticle having a special-purpose measurement mark is set on the exposure apparatus, and exposure (test print) on the photosensitive substrate is performed while slightly changing conditions so as to obtain an optimal focal point position and a proper exposure amount. The pattern formed on the photosensitive material is observed using an optical microscope or a scanning type electron microscope to obtain optimal conditions (see U.S Pat. No. 4,908,656). In this case, the above-mentioned operation need not be performed for a reticle or a photosensitive substrate whose optimal conditions have already been known.
Thereafter, upon execution of exposure on an actual photosensitive substrate, the exposure amount is controlled using an integrator sensor comprising a photoelectric conversion element arranged in an illumination optical system of the exposure apparatus. The integrator sensor is arranged at a position almost conjugate with an exit surface side (secondary light source forming surface) of a fly-eye lens as an optical integrator arranged in the illumination optical system, and some light components of exposure light emerging from the fly-eye lens and radiated toward the photosensitive substrate are guided toward the integrator sensor by a beam splitter. In this case, the level of a photoelectric conversion signal of the integrator sensor is set in advance in correspondence with exposure energy incident on the photosensitive substrate. During exposure, the photoelectric conversion signal from the integrator sensor is integrated, and accumulated exposure energy on the photosensitive substrate is calculated based on the integrated value. When the accumulated exposure energy reaches a proper exposure amount, exposure light is shielded by, e.g., a shutter, thereby controlling the exposure amount onto the photosensitive substrate.
In the conventional normal illumination optical system, since the photoelectric conversion signal from the integrator sensor and the exposure energy on the photosensitive substrate have an almost constant, simple relationship therebetween, a good result can be obtained by the conventional exposure amount control method.
In recent years, circuit patterns of LSIs are further miniaturized, and demand for exposing a finer pattern on a photosensitive substrate with a high resolution is increased. As one of methods for meeting this demand, the numerical aperture (N.A.) of a projection optical system is increased. However, since the increase in N.A. is limited, the following various attempts have been made recently.
In a method called a phase shift method disclosed in, e.g., Japanese Patent Publication No. 62-50811, exposure is performed using a phase shift reticle prepared by forming a phase film on a light transmission portion sandwiched between chromium patterns so as to shift the phase of light transmitted through a specific portion of the reticle. With this phase shift method, a value as a coherence factor of an illumination optical system largely influences imaging performance, and in order to image a finer pattern, the a value is set to be a relatively small value (e.g., about 0.1 to 0.4).
An actual semiconductor element is normally formed by forming 20 layers or more circuit patterns to overlap each other. Since the circuit patterns of these layers require different resolutions, both normal reticles and phase shift reticles are often used. Therefore, as disclosed in, e.g.,U.S. Pat. No. 4,931,830, an exposure apparatus having an illumination optical system with a variable .sigma. value is required. As a method of changing the .sigma. value, a method of arranging a variable aperture stop near an exit surface of a fly-eye lens in the illumination optical system, i.e., a Fourier transform plane (pupil plane) of the illumination optical system with respect to a reticle pattern is known. When the .sigma. value is changed in this manner, the amount of light incident on the integrator sensor also changes, and exposure amount control is made based on the changed amount of light.
Furthermore, in recent years, in order to allow projection exposure with a high resolution and a large focal depth, a method called a zone illumination method (see Japanese Laid-Open Patent Application No. 61-91662) or a modified light source method (see Japanese Laid-Open Patent Application No. 4-268715, U.S. Serial No. 847,030 filed Apr. 15,1992, now abandoned, U.S. Ser. No. 791,138 filed Nov. 13, 1991, now abandoned and SPIE "Optical/Laser Microlithography V", 1992,Vol. 1674) has been proposed. In the zone illumination method, the distribution of a secondary light source (i.e., the light amount distribution of illumination light) on the Fourier transform plane of the illumination optical system with respect to a reticle pattern has a ring shape. On the other hand, in the modified light source method, the light amount distribution of illumination light on the Fourier transform plane of the illumination optical system with respect to a reticle pattern becomes maximal at least at one position separated from the optical axis by an amount corresponding to the degree of micropatterning (e.g., the pitch) of the reticle pattern. In other words, illumination light passing through the Fourier transform plane is limited to at least one partial region decentered from the optical axis.
Therefore, in either illumination method, the distribution state (light amount distribution of illumination light) and the coherence factor of illumination light on the Fourier transform plane of the illumination optical system with respect to a reticle pattern are different from those of a normal illumination method. Therefore, incident conditions (the intensity, the incident angle, the angle range, and the like of illumination light) of illumination light incident on the integrator sensor also change.
The same applies to a radiation amount monitor, placed on a stage, for measuring exposure energy of illumination light onto a wafer. This will be explained below with reference to FIG. 4.
FIG. 4 shows a schematic arrangement of the above-mentioned conventional reduction production type exposure apparatus (stepper). In FIG. 4, illumination light L1 from a light source (not shown) is collimated into a parallel light beam, and the parallel light beam is incident on a fly-eye lens 1. The illumination light L1 emerging from the fly-eye lens 1 is almost vertically incident on a pattern 5 on a reticle R via a spatial filter (aperture stop) 3 and a condenser lens 4. An image of the pattern 5 on the reticle R is formed on the best imaging plane by a projection optical system PL.
The spatial filter 3 is arranged on or near a reticle-side focal plane 2, i.e., the Fourier transform plane (to be referred to as "pupil plane" hereinafter) of the fly-eye lens 1 with respect to the pattern 5 on the reticle R. The spatial filter 3 has an almost circular or rectangular aperture having an optical axis AX of the projection optical system PL as a center, and limits a two-dimensional secondary light source image formed in the pupil plane to a circular (or rectangular) shape. A wafer stage 8 is movably arranged below the projection optical system PL, and a wafer W is held on the wafer stage 8. A radiation amount monitor 7 comprising a photoelectric conversion element is fixed near the wafer W on the wafer stage 8, so that the light-receiving surface of the monitor 7 is arranged at the same level as that of the exposure surface of the wafer W.
The wafer stage 8 is constituted by an XY stage for positioning the wafer W and the radiation amount monitor 7 in a plane perpendicular to the optical axis AX of the projection optical system PL, a Z stage for positioning the wafer W and the radiation amount monitor 7 in the direction of the optical axis AX, and the like. When exposure is performed on the wafer W, the wafer W is set in the exposure region of the projection optical system PL, and the image of the pattern 5 on the reticle R is formed and exposed on each shot region on the wafer W. When exposure energy of illumination light onto the wafer W is to be measured, the wafer stage 8 is driven to set the light-receiving surface of the radiation amount monitor 7 in the exposure region of the projection optical system PL, and the image of the pattern 5 on the reticle R is projected onto the light-receiving surface of the radiation amount monitor 7. The output signal from the radiation amount monitor 7 is supplied to a signal processing device 9. The light-receiving surface of the radiation amount monitor 7 is set to be perpendicular to the optical axis AX of the projection optical system PL, and illumination light is almost vertically incident on the light-receiving surface of the radiation amount monitor 7. The illumination light L1 incident on the reticle R falls within a predetermined incident angle range having the optical axis AX as the center, and +1st-order diffracted light Dp, -1st-order diffracted light Dm, and the like emerge from the pattern 5 on the reticle R in addition to 0th-order diffracted light D0. Therefore, the incident angle of the illumination light incident on the radiation amount monitor 7 is distributed within a predetermined range to have 0 as the center. More specifically, the average value of incident angles of the illumination light incident on the radiation amount monitor 7 is 0.
Since the diameter of the circular aperture of the spatial filter 3 shown in FIG. 4 is constant, the signal processing device 9 multiplies a photoelectric signal output from the radiation amount monitor 7 with a predetermined conversion coefficient, thus calculating the amount of radiation energy radiated on the wafer W.
However, in an illumination optical system whose value is variable within a range from 0.1 to 0.8, if the conversion coefficient in the signal processing device 9 is constant, the amount of radiation energy calculated from the level of the photoelectric signal suffers from an error. For example, when the .sigma. value is changed from 0.6 to 0.2, if the ratio between the two amounts of radiation energy obtained by the signal processing device 9 almost coincides with the ratio between .sigma. values, i.e., the area ratio of the aperture in the pupil plane of the illumination optical system (almost corresponding to the ratio of a light source image present in the aperture), a precise amount of radiation energy can be calculated regardless of the .sigma. value. However, when the .sigma. value is changed, incident conditions (the incident angle, the range of the incident angle, and the like) of illumination light with respect to the radiation amount monitor 7 also change. For this reason, even if a precise amount of radiation energy is obtained when the .sigma. value is 0.2, the amount of radiation energy calculated when the G value is 0.6 is not always precise, i.e., does not always coincide with the amount of light actually incident on the radiation amount monitor 7. More specifically, in an illumination optical system with a variable .sigma. value, a precise amount of radiation amount (illumination light intensity) cannot be obtained by the radiation amount monitor or integrator sensor in units of conditions. This problem similarly occurs not only in the illumination optical system with a variable o value but also in an illumination optical system which can adopt the zone illumination method or modified light source method. For example, the same problem is posed in the zone illumination method when the inner or outer diameter or the zone ratio (ratio between the inner and outer diameters) of a ring-shaped region is changed; in the modified light source method when the positions where the light amount distribution of illumination light in the pupil plane of the illumination optical system becomes maximal are changed or the number of such positions is changed; and when the normal illumination method, the zone illumination method, and the modified light source method are selectively used as needed.
As described above, when the illumination conditions for a reticle are changed, in other words, when the light amount distribution of illumination light in the pupil plane of the illumination optical system is changed, the incident conditions of illumination light incident on the integrator sensor or the radiation amount monitor also change. For this reason, even when the intensity of illumination light incident on the integrator sensor remains the same before and after the illumination conditions are changed, the level of a photoelectric signal output from the sensor changes. That is, the intensity (or amount of radiation energy) of illumination light radiated onto the sensor cannot be precisely measured. In a projection exposure apparatus, exposure control is performed using the output signal from the integrator sensor. In this case, exposure amount control precision is lowered for the above-mentioned reason, and a reticle pattern cannot be precisely exposed on a photosensitive substrate.
Even when a reticle is changed from a normal reticle to a phase shift reticle (or vice versa) without changing the illumination conditions, the incident conditions of illumination light emerging from the projection optical system and incident on the radiation amount monitor change. Furthermore, as the phase shift reticle, a spatial frequency modulation type reticle, an edge emphasis type reticle, a shifter light-shielding type reticle, a halftone type reticle and the like have been proposed. If phase shift reticles of different types are used, the incident conditions of illumination light on the radiation amount monitor change. Therefore, the intensity (or amount of radiation energy) of illumination light cannot be precisely measured especially by the radiation amount monitor even when the reticle type is changed.